LONDON — It was early one Sunday morning when the killer rang at a front door that was decorated with a wreath for Christmas. When his victim answered, he fired four fatal shots, ran off and disappeared.

The unsolved murder of Marianne Wilkinson, 68, in a town near Dallas, Texas, in December 2007 has left investigators with few clues and few leads. They recovered the gun used in the crime a few months later, but the gunman's identity -- and his motive -- remain a mystery.